Positive displacement rotary gas meters generally consist of a counter module and a gas meter pressure body. The gas meter pressure body converts gas flow into rotational motion via two solid impellers, which sweep out a known volume on each rotation. The impellers are precisely linked together using timing gears such that they rotate in a desired configuration. As a result of their fixed orientation and swept volumes, when the impellers are driven by a gas flow, the impellers only allow a set volume of gas to pass through the meter on each rotation. Accordingly, the quantity of gas passing through the meter can be calculated by counting the number of rotations of at least one of the impellers. Counting impeller rotations is commonly done by configuring the gas meter such that a mechanical or electronic counter module is driven by at least one of the impeller shafts via the use of a magnetic coupling module.
However, gas flow through a meter may become interrupted or temporarily reversed during normal operation of the gas meter, which can lead to temporary reversal of gas flow through the meter. Longer term reversal of gas flow can also result when a gas meter is improperly installed within a gas distribution system. Any such flow interruption or reversal can drive the impellers to rotate in a direction opposite their intended rotational direction. Opposite rotation of the impellers can result in the reverse rotation of the magnetic coupling within the magnetic housing and of the mechanical or electrical counter. The reversal of the mechanical or electrical counter will result in an inaccurate counting of the gas flow volume that has passed through the meter. There is therefore a need to design a rotary gas meter that will not be susceptible to these known types of counting.